Force timeout_time to be int.

Fix type error I got from having `MENTOS_TIMEOUT` set:

```console
/Users/chen/.rubies/ruby-2.1.2/lib/ruby/2.1.0/timeout.rb:76:in `timeout': undefined method `zero?' for "100":String (NoMethodError)
	from /Users/chen/.gem/ruby/2.1.2/gems/pygments.rb-0.6.0/lib/pygments/popen.rb:220:in `mentos'
$ echo $MENTOS_TIMEOUT
100
```